A High Electron Mobility Transistor (HEMT) is a field-effect transistor incorporating a heterojunction between two materials with different energy bandgaps as the channel instead of a doped region. In particular, Aluminum gallium nitride/gallium nitride (AlGaN/GaN), indium aluminum nitride/GaN (InAlN/GaN) Al/GaN or AlGaN/InGaN/GaN based HEMTs have received increasing attention for high power and high frequency applications such as military radar and satellite-based communications systems, due to their superior mobility (˜1400 cm2/V-s) and larger energy bandgap as compared to Si-based power transistors.
Regarding the GaN epi-layer, due to the lack of readily available, large-area GaN bulk materials, GaN epi-layers are usually grown on sapphire, silicon (Si) or silicon carbide (SiC) substrates. Among these substrates used for GaN epi-layers, sapphire is the most common one and is mainly used for GaN-based light emitting diode growth. However, the thermal conductivity of GaN is very poor, impacting the electronic device performance and reliability especially for high power applications. SiC substrates are an excellent choice for epitaxy growth owing to their high thermal conductivity and smaller lattice mismatch to GaN, but they are very expensive. Si is another common substrate because of the low cost, availability of large area wafers, relatively high thermal conductivity, and mature Si-based processing techniques. Recently, HEMT structures grown on 8-inch Si substrates have demonstrated that Si substrates are a prime candidate for commercialized, GaN mass-production applications. However, the large lattice mismatch between Si and GaN makes the nucleation interface layer become defective, requiring a thicker transition to grow good quality GaN epi-layers.